The present disclosure is related to the field of X-ray imaging. More specifically, the present disclosure relates to dynamic damping of high voltage input power to an X-ray source.
In conventional computed tomography (CT) X-ray imaging systems, an X-ray source emits a cone-shaped X-ray beam toward a subject or object, such as a patient or piece of luggage. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the X-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the X-ray intensity received by that particular detector element. The electrical signals are quantized and transmitted to a data processing system for analysis, which generally results in the presentation of an image.
CT imaging systems may comprise energy-discriminating (ED), multi-energy (ME), and/or dual-energy (DE) CT imaging systems that may be referred to as an EDCT, MECT, and/or DECT imaging system. The EDCT, MECT, and/or DECT imaging systems are configured to measure energy-sensitive projection data. The energy-sensitive projection data may be acquired using multiple applied X-ray spectra by modifying the operating voltage of the X-ray tube or utilizing X-ray beam filtering techniques (e.g., energy-sensitive X-ray generation techniques), or by energy-sensitive data acquisition by the detector using energy-discriminating, or with photon counting detectors or dual-layered detectors (e.g., energy-sensitive X-ray detection techniques).
With X-ray generation techniques, various system configurations utilize modification of the operating voltage of the X-ray tube including: (1) acquisition of projection data from two sequential scans of the object using different operating voltages of the X-ray tube, (2) acquisition of projection data utilizing rapid switching of the operating voltage of the X-ray tube to acquire low-energy and high-energy information for an alternating subset of projection views, or (3) concurrent acquisition of energy-sensitive information using multiple imaging systems with different operating voltages of the X-ray tube.
EDCT/MECT/DECT provides energy discrimination capability that allows material characterization. For example, in the absence of object scatter, the system utilizes signals from two applied photon spectra, namely the low-energy and the high-energy incident X-ray spectrum. The low-energy and high-energy incident X-ray spectra are typically characterized by the mean energies of the applied X-ray beams. For example, the low-energy X-ray spectrum comprises X-ray photons with lower-energy photons, resulting in a lower mean energy, relative to the high-energy X-ray spectrum. The detected signals from low-energy and high-energy X-ray spectra, either from two different applied spectra (X-ray generation techniques) or by regions of the same applied spectrum (X-ray detection techniques) provide sufficient information to estimate the effective atomic number of the material being imaged. Typically, X-ray attenuation mechanisms (Compton scattering or Photoelectric absorption) or the energy-sensitive attenuation properties of two basis materials (typically water and calcium for patient scanning) are used to enable estimation of the effective atomic number.
Dual-energy scanning can obtain diagnostic CT images that enhance contrast separation within the image by utilizing energy-sensitive measurements. To facilitate processing of the energy-sensitive measurements, the applied X-ray spectrum should be constant during an integration period. For example, such CT systems that acquire interleaved subsets of low-energy and high-energy projection data (versus two separate scans) should operate to maintain the accelerating voltage steady during an acquisition interval. Also, the change from one voltage level to another voltage level should occur very rapidly. Less stable X-ray tube operating voltages and/or slower operating voltage switching times result in a reduction in the difference in effective mean energy (the average of the mean energy of time-varying X-ray spectrum) of the applied X-ray spectra, which reduces the fidelity of the system in characterizing different materials.
The term “tube-spit” refers to temporary electrical short-circuits that sometimes occur inside an x-ray tube. Typically, upon the occurrence of tube-spit, the supply of power to the x-ray tube is temporarily interrupted to prevent arcing. Power is restored to the tube after a time interval of, for example, about one millisecond. During tube-spit recovery, no x-ray photon is emitted from the x-ray tube. As a result, detector measurements taken during the recovery are invalid.